This invention relates generally to vortex-shedding flowmeters, and more particularly to a flow meter of this type which includes a shedder producing vortices and a sensor torsionally-mounted within the flow pipe by a torque tube which is coupled by a sensor link assembly to an external torque transducer to generate an output signal whose frequency is proportional to the flow rate of the fluid being metered, the flowmeter being capable of accurately measuring the flow rates of liquids or gases even under extreme operating conditions.
It is well known that under certain circumstances the presence of an obstacle or shedder in a flow pipe will give rise to periodic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed which are known as Karman vortex streets. The frequency at which these vortices are shed is a function of flow rate.
This phenomenon is exploited to create a flowmeter for measuring the volumetric flow of fluids being treated or supplied in order to carry out various control functions. Flowmeters operating on this principle are disclosed in the Bird U.S. Pat. No. 3,116,639, and in the White U.S. Pat. No. 3,650,152. Flowmeters of the vortex-shedding type, such as those disclosed in the Burgess U.S. Pat. No. 3,888,120 and the Herzl U.S. Pat. No. 4,162,238, are capable of effecting accurate volumetric or mass flow measurement.
The above-identified copending patent application (C), whose entire disclosure is incorporated herein by reference, discloses a vortex-type flowmeter in which fluidic oscillations produced by a shedder mounted in a flow pipe are sensed by a downstream balanced-vane sensor pivoted in a torsional suspension that allows only microscopic vane motion. The shedder acts to divide the incoming fluid flowing therethrough and causes vortices to be shed alternately on either side thereof. The downstream train of vortices passing on either side of the vane sensor generates fluidic forces giving rise to alternate clockwise and counterclockwise torques, causing the sensor to oscillate mechanically at a frequency proportional to the flow rate of the fluid being metered.
The above-identified copending patent application (B), whose entire disclosure is incorporated herein by reference, discloses a vortex-shedding flowmeter wherein torsionally-supported behind the shedder is a drag-actuated sensor which includes a pair of parallel legs symmetrically disposed with respect to the longitudinal axis of the flow pipe.
With a drag-actuated sensor, as vortices are successively detached from the shedder and appear alternately on either side of the gap between the shedder and the downstream sensor, the low pressure region generated by each vortex acts to displace the stagnant zone produced in this gap as a result of fluid flow past the shedder to a position in front of the adjacent leg of the sensor, the fluid flow then going around and past the other leg, thereby developing a torque about the pivot axis. These torques are developed alternately, causing the torsionally-supported sensor to oscillate at a frequency in accordance with flow rate.
In both patent applications (B and C), the oscillatory motion of the torsionally-supported sensor is detected by means of a transducer which takes the form of a strain gauge bonded to a resilient beam, one end of which is attached to the trunnion or shaft of the sensor projecting through the flow pipe, the other end being anchored. The resultant deformation of the beam as the shaft oscillates is translated by the strain gauge into a corresponding electrical signal whose frequency is indicative of flow rate.
As pointed out in the earlier-filed copending patent applications, an important advantage of a vortex flowmeter having a torsionally-mounted sensor is that the meter is effective and accurate for both liquid and gas flow measurements. Though the vortex-type flowmeters disclosed therein represent a signficant advance over prior art vortex-type meters, such as those disclosed in the above-identified patents, their torque transducer arrangements have certain drawbacks and therefore fall short of an ideal arrangement.
The torque transducer arrangement disclosed in the above-identified patent application (A) closely approaches the ideal requirements for a sensing system constituted by a torque transducer associated with a torsionally-mounted sensor in a vortex-type flowmeter. These ideals are as follows.
A. The system has a sensitivity which renders the meter effective for low-pressure gas measurement.
B. The system is one which has an inherent ruggedness that renders the meter suitable for heavy-duty liquid flow rate measurement.
C. The system is insensitive to mechanical vibration and shock and acceleration forces to which the flowmeter is subjected.
D. The system is capable of operating over the broad temperature range normally encountered in gas and liquid measurement and is capable of operating over a very wide operating frequency range.
E. The sensing system requires virtually no motion and is not limited by torque transducer bonding or attachment problems.
F. Finally, the sensing system is one which is relatively inexpensive and has a compact structure.
In the torque transducer arrangement disclosed in application (A), the transducer cooperates with an extension of the shaft on which the torsionally-mounted sensor of the vortex meter is pivoted, the shaft extension having two flat parallel faces on opposing sides thereof. The transducer assembly is constituted by a first pair of parallel-connected piezoelectric elements lying in a common plane and interposed between one face of the shaft extension and a first pre-loading block, and a second pair of parallel-connected piezoelectric elements lying in a common plane and interposed between the opposite face of the shaft extension and a second pre-loading block, the movement of the extension being restricted by the pre-loaded elements to a degree whereby the extension is virtually motionless. The two pairs of parallel-connected elements are connected to output terminals and are so polarized in relation to the faces in the shaft extension that alternate clockwise and counterclockwise torques cause the interconnected elements to generate an alternating voltage of the same frequency.
In the vortex-shedding flowmeters disclosed in my earlier-filed applications (A), (B) and (C), the sensor in each instance is torsionally mounted on a shaft projecting through a bore in the flow pipe, the free end of the shaft being operatively coupled to an external transducer to convert the oscillatory motion of the sensor into a corresponding electrical signal. In order to prevent fluid leakage through the bore, the shaft therethrough is provided with an elastomeric seal made of neoprene or a material having similar elastomeric and physical properties.
While an elastomeric seal functions effectively under normal operating conditions even when corrosive fluids are being metered which are either very hot or very cold, it is not an acceptable seal under extreme conditions. Thus an elastomeric seal may break down under extremely high temperature operating conditions, such as those encountered with steam or liquid salts, or under extremely low temperature conditions involving cryogenic liquids such as liquid nitrogen or oxygen, or liquefied natural gas.